In many technical fields, mixtures of materials are used in order to control or improve certain properties of a product. Such material blends may be, e.g. in the form of loose mixtures, or in the form of composite structures.
A composite material is basically a combination of two or more materials, each of which retains its own distinctive properties. The resulting material has characteristics that are not characteristic of the components in isolation. Most commonly, composite materials have a bulk phase, which is continuous, called the matrix; and a dispersed, non-continuous, phase called the reinforcement. Some other examples of basic composites include concrete (cement mixed with sand and aggregate), reinforced concrete (steel rebar in concrete), and fibreglass (glass strands in a resin matrix).
The following are some of the reasons why composites are selected for certain applications:                High strength to weight ratio (low density high tensile strength)        High creep resistance        High tensile strength at elevated temperatures        High toughness        
Typically, reinforcing materials are strong, while the matrix is usually a ductile, or tough, material. If the composite is designed and fabricated correctly, it combines the strength of the reinforcement with the toughness of the matrix to achieve a combination of desirable properties not available in any single conventional material. For example: polymer/ceramic composites have a greater modulus than the polymer component, but are not as brittle as ceramics.
Since the reinforcement material is of primary importance in the strengthening mechanism of a composite, it is convenient to classify composites according to the characteristics of the reinforcement. The following three categories are commonly used:    a) “fibre reinforced”, wherein the fibre is the primary load-bearing component.    b) “particle reinforced”, wherein the load is shared by the matrix and the particles.    c) “dispersion strengthened”, wherein the matrix is the major load-bearing component.    d) “structural composites”, wherein the properties depend on the constituents, and the geometrical design.
Generally, the strength of the composite depends primarily on the amount, arrangement and type of fibre (or particle) reinforcement in the resin. In addition, the composite is often formulated with fillers and additives that change processing or performance parameters.
Thus, in the prior art, it is generally known to combine different materials in order to obtain materials having modified properties or being able to control certain properties of a material to which they are applied, and there is a continuous need for such materials allowing for the tailor-made control of material characteristics, as well as regarding their cost-efficiency and environmental compliance.
An important field in this respect is the production of structured material and their properties.
One example of structured materials is paper, in the manufacture of which a number of different materials are combined, each of which can positively or negatively influence the properties of the other components, or the final paper.
One of the most common groups of additives in the field of paper manufacturing and finishing are fillers having several advantageous functions in paper. For example, fillers are used for reasons of opacity or the provision of a smoother surface by filling the voids between the fibres.
There are, however, limitations with respect to the amount of fillers, which can be added to the paper, as increasing filler amounts in conventional paper leads to an inverse relationship between the strength and optical properties.
Thus, conventional paper may contain a certain amount of fillers, but if the filler content is too high, the mechanical properties of the paper will significantly decrease.
Several approaches have been proposed to improve this relationship and to produce a highly filled paper having good optical as well as mechanical properties, but there is still a need for processes for manufacturing paper allowing for a higher filler content as commonly used without essentially impairing the paper strength.
Searching for methods for controlling the properties of structured materials or of products containing such structured materials, it was found that special nano-fibrillar cellulosic gels comprising calcium carbonate can be useful.
Cellulose is the structural component of the primary cell wall of green plants and is the most common organic compound on Earth. It is of high interest in many applications and industries.
Cellulose pulp as a raw material is processed out of wood or stems of plants such as hemp, linen and manila. Pulp fibres are built up mainly by cellulose and other organic components (hemicellulose and lignin). The cellulose macromolecules (composed of 1-4 glycosidic linked β-D-Glucose molecules) are linked together by hydrogen bonds to form a so called primary fibril (micelle) which has crystalline and amorphous domains. Several primary fibrils (around 55) form a so called microfibril. Around 250 of these microfibrils form a fibril.
The fibrils are arranged in different layers (which can contain lignin and/or hemicellulose) to form a fibre. The individual fibres are bound together by lignin as well.
When fibres become refined under applied energy they become fibrillated as the cell walls are broken and torn into attached strips, i.e. into fibrils. If this breakage is continued to separate the fibrils from the body of the fibre, it releases the fibrils. The breakdown of fibres into microfibrils is referred to as “microfibrillation”. This process may be continued until there are no fibres left and only fibrils of nano size (thickness) remain.
If the process goes further and breaks these fibrils down into smaller and smaller fibrils, they eventually become cellulose fragments or nano-fibrillar gels. Depending on how far this last step is taken some nano-fibrils may remain amongst the nano-fibrillar gels. The breakdown to primary fibrils may be referred to as “nano-fibrillation”, where there may be a smooth transition between the two regimes. The primary fibrils form in an aqueous environment a gel (meta stable network of primary fibrils) which may be referred to as “nano-fibrillar gel”. The gel formed from the nano-fibrils can be considered to contain nanocellulose.
Nano-fibrillar gels are desirable as they usually contain very fine fibrils, considered to be constituted in part of nanocellulose, showing a stronger binding potential to themselves, or to any other material present, than do fibrils which are not so fine or do not exhibit nanocellulosic structure.
From unpublished European patent application No. 09 156 703.2, nano-fibrillar cellulose gels are known. However, there is no teaching with respect to their effects in structured materials.